It would be in Apple's best interests to lend engineering support to major vendors to help them get their products compatible with the latest-and-greatest.
I'd love to upgrade to Mountain Lion, but I won't until I can shoot tethered. I'm sure I'm far from the only one. And those who shoot tethered are "creative" types who spend lots of money on tech stuff...one would think they'd be the ones Apple would count on the most to be early evangelists....
That's part of the risk of being on the bleeding edge - sometimes we bleed. This is especially the case with software and hardware that focus on vertical markets.
As far as media and content creation - pro audio, video editing, and yes photo, you really need to be sure that the tools work before upgrading. They will work eventually, but if you wan to "play" right out of the gate - have a spare machine to experiment with.
Mountain Lion, as Lion did, breaks many things: final cut studio, Protools, quickbooks, just a few that have proven themselves DOA during testing. So a production machine needs to be just that - and not a test bed.
Sorry for the rant - I just had the same conversation with one of our younger editors.
As far as canon is concerned - have they fixed the 1Dx osx tether issue??